The problem of monitoring wear of system components in a more or less automated way has ever increasing importance in many fields of application. It is known to provide system components with sensors which can be read out for purposes of maintenance and for deciding whether a system component needs replacement or not.
This is of particular relevance in the field of bearings, because bearing failure may lead to important damages or system failures well beyond the damage of the bearing itself in many fields of application.
In these fields of application where system-relevant components are subject to wear and failure can lead to major damages, the maintenance schedule has to be such that the components are replaced well before the risk of failure starts to increase when approaching the end of the component lifetime. However, replacement may require system shutdown and may be quite expensive such that it is very important to define the replacement cycles in such a way that a good balance between reduction of risk of failure on the one hand and low system downtime and replacement or maintenance costs on the other hand is reached.
One example is a typical helicopter rotor control (self-lubricating) bearing. In the case of helicopter blade pitch, bearings are inspected by measurement of the clearance between the inner and outer ring.
Each bearing is measured individually to check for clearance evolution. This is almost always done with the bearing disassembled from the aircraft. This clearance is then used to estimate the remaining self-lubricating material (liner); the flight safety is maintained through tracking this clearance evolution at disassembly intervals, and pre-flight visual inspections.
This measurement can be inconsistent due to:                contamination/liner debris;        unequal wear from an offset reversing load (can result in premature bearing replacement as remaining liner thickness cannot be established);        human error; and        the usable lubricating material being relatively thin (˜150 μm)        
Dynamic measurements (e.g. strain, temperature) are rarely or not applied at all in this field of application. However, state of the art for bearing condition monitoring, if applied, would focus on bearing readings and comparison to expected readings from reference data gathered in calibration for the given combination of bearing and application. This method will typically require testing and calibration for each specific system and application. Calibration and wear prediction may be difficult or impossible in cases where the field of application, or its environment is rapidly changing in an unpredictable way. This is the case e.g. for aircraft components, which are used under widely varying environmental conditions.
Current solutions suffer from the drawbacks that the frequency of inspection intervals may be high, early product replacement may increase costs, whereas dynamic measurements of individual components may be difficult to calibrate.